Normally and in the context of the present disclosure, the air ratio λ is defined as the ratio of the air mass mAir,actual actually fed to the at least one cylinder of the internal combustion engine to the stoichiometric air mass mAir,stoic that would be just enough to oxidize completely the fuel mass mFuel fed to the at least one cylinder (stoichiometric operation of the internal combustion engine λ=1). The following applies: λ=mAir,actual/mAir,stoic and, with the stoichiometric air requirement Lstoic, which is defined by Lstoic=mAir,stoic/mFuel, the air ratio is given by λ=mAir,actual/mFuel*(1/Lstoic).
Internal combustion engines are fitted with various exhaust gas aftertreatment systems to reduce pollutant emissions. In the case of spark ignition engines, catalytic reactors are employed, using catalytic materials which increase the rate of certain reactions to ensure oxidation of HC and CO, even at low temperatures. If nitrogen oxides NOx are additionally to be reduced, this can be achieved by the use of a three-way catalyst, although this utilizes stoichiometric operation (λ≈1) of the spark ignition engine within narrow limits. In this case, the nitrogen oxides NOx are reduced by means of the available non-oxidized exhaust gas components, namely the carbon monoxides CO and the unburned hydrocarbons HC, and the exhaust gas components are simultaneously oxidized.
In the case of internal combustion engines which are operated with an excess of air, e.g. lean burn spark ignition engines and direct injection diesel engines, as well as direct injection spark ignition engines, the nitrogen oxides NOx in the exhaust gas cannot be reduced owing to the absence of reducing agents.
As a result, an exhaust gas aftertreatment system may be provided in order to reduce the nitrogen oxides, e.g. a storage catalyst, which is also referred to as an LNT (Lean NOx Trap). In this case, the nitrogen oxides are absorbed, e.g. collected and stored, in the catalyst during a lean-mixture operating mode (λ>1) of the internal combustion engine, and are then reduced during a regeneration phase with a substoichiometric operating mode (λ<1) of the internal combustion engine with a deficiency of oxygen, wherein the unburned hydrocarbons HC and the carbon monoxide CO in the exhaust gas serve as reducing agents. Further possibilities within the engine for enriching the exhaust gas with reducing agent, in particular with unburned hydrocarbons, are offered by exhaust gas recirculation (EGR) and, in the case of diesel engines, throttling in the intake section. Enrichment of the exhaust gas with unburned hydrocarbons can also be achieved by an afterinjection of fuel into at least one cylinder of the internal combustion engine. One disadvantage of the latter mentioned procedure is, in particular, dilution of the oil. It is possible to introduce hydrocarbons directly into the exhaust section, e.g. by injection of additional fuel upstream of the LNT, thereby dispensing with injection into the cylinder itself.
During the regeneration phase, the nitrogen oxides (NOx) are released and are substantially converted into nitrogen dioxide (N2), carbon dioxide (CO2) and water (H2O). The temperature of the storage catalyst may preferably be within a temperature window between 200° C. and 450° C., on the one hand ensuring rapid reduction and, on the other hand, preventing desorption without conversion of the nitrogen oxides NOx that are released again, something that can be triggered by excessive temperatures.
One difficulty in using a storage catalyst results from the sulfur contained in the exhaust gas, which is likewise absorbed and which has to be removed at regular intervals in a process referred to as desulfurization. For this purpose, the storage catalyst may be heated to high temperatures, generally between 600° C. and 700° C., and supplied with a reducing agent, e.g. unburned hydrocarbons. The high temperatures utilized for desulfurization may damage the storage catalyst, contribute to thermal aging of the catalyst and significantly reduce the desired conversion of nitrogen oxides toward the end of its life.
The storage capacity or ability to store nitrogen oxides decreases as the time in operation of the LNT increases, this being attributable to the contamination of the storage catalyst with sulfur, e.g. to the accumulation of sulfur, and also to thermal aging due to the high temperatures.
In addition to regeneration, e.g. cleaning of the LNT, which may be carried out at regular intervals, and desulfurization, the low nitrogen oxide emission limits specified by law may in future require onboard diagnosis (OBD) in order to monitor or detect the limitation in ability to function, e.g. the decrease in conversion, that can be expected as the time in operation of the LNT increases.
The technical relationships described above describe the advantages of methods for substoichiometric operation of an internal combustion engine in order to clean and desulfurize an LNT. On the other hand, however, also methods may be advantageous for monitoring the ability to function of the LNT to ensure that undesirably high pollutant emissions due to a limited ability to function or lack of conversion are reliably avoided.
Transient operating conditions make it considerably more difficult to maintain a constant air ratio and, in isolated cases, may even make it impossible since it is not possible to follow the input by the driver via the gas pedal without a delay, and especially because the operating parameters that determine the air ratio, namely the air mass and fuel quantity, can be adjusted and adapted to the new operating conditions with a delay and at different speeds.
In the range of relatively high, high and maximum loads (see FIG. 2—range 202), initiation and maintenance of a substoichiometric operating mode is generally governed by the maximum permissible exhaust gas temperature, with the exhaust gas temperature often being limited by components provided in the exhaust gas discharge system or by the thermal load bearing capacity of said components, e.g. by the turbine of an exhaust gas turbocharger, an exhaust gas aftertreatment system or the exhaust gas recirculation system. In this context, it may be taken into account that the exhaust gas temperature generally increases when the mixture is enriched.
As regards the methods for monitoring or checking the ability to function of a storage catalyst, it may be stated that these methods likewise often utilize a substoichiometric operating mode of the internal combustion engine. Here, maintaining a constant or substantially constant air ratio λ is of decisive importance.
European Patent Application EP 1 936 140 A1 describes a method for monitoring a storage catalyst using two lambda probes, or oxygen sensors, in which a measuring error of the lambda probes is exploited. More specifically, if the unburned hydrocarbons in the exhaust gas exceed a certain concentration, the probe outputs a higher value for the air ratio λmeas than is actually present, e.g. an air ratio of λmeas=0.95 as a measured variable in the case of a substoichiometric operating mode (λ<1) of the internal combustion engine and an HC concentration of 10,000 ppm in the exhaust gas, even though the air ratio is actually λactual=0.85.
To check the ability to function of the storage catalyst, the HC concentration in the exhaust gas is deliberately increased in such a way that the first probe, which is arranged upstream of the storage catalyst, operates incorrectly. If the storage catalyst is not capable of functioning properly, e.g. the storage capacity is at least limited, no more unburned hydrocarbons or less unburned hydrocarbons are oxidized by the release of nitrogen oxide NOx and the HC concentration downstream of the storage catalyst is essentially exactly the same as upstream of the storage catalyst, for which reason both lambda probes output the same value—subject to a measurement error of equal magnitude—for the air ratio. The storage catalyst is therefore assumed to be incapable of functioning properly if the air ratios λ1,meas, λ2,meas determined by means of probes are of substantially equal magnitude and λ1,meas/λ2,meas≈1.
If, on the other hand, the storage catalyst is still capable of functioning properly, the unburned hydrocarbons in the exhaust gas are at least partially oxidized in the storage catalyst as they flow through, for which reason the HC concentration in the exhaust gas downstream of the storage catalyst will be lower than upstream of the catalyst. Thus, the storage catalyst will be assumed to be at least partially capable of functioning if the two air ratios λ1,meas, λ2,meas are of different magnitudes with λ1,meas/λ2,meas>1. Here, the air ratio λ2,meas determined with the second probe, which is arranged downstream of the storage catalyst, does not necessarily have to be free from measurement error. However, the deviation of the air ratio λ2,meas from the actual air ratio λ is at least less than upstream of the storage catalyst.
The method described in EP 1 936 140 A1 is dependent on the maintenance of a constant or substantially constant air ratio λ and requires steady-state operation of the internal combustion engine.
The German patent application with the file reference 102012200006.2 likewise describes a method for monitoring a storage catalyst in which respective lambda probes for detecting the air ratio λ are arranged upstream and downstream of the storage catalyst. To check the ability to function of the storage catalyst, the internal combustion engine is switched to a substoichiometric operating mode (2<1) for a specifiable period of time, in which mode however—in contrast to the method described in EP 1 936 140 A1—both probes operate without error. The method can also be carried out in the non-steady-state operating mode of the internal combustion engine but also requires the maintenance of a constant or substantially constant air ratio λ.
The inventors recognize the aforementioned disadvantages and herein disclose systems and methods for initiating and maintaining a specifiable substoichiometric (λ<1) operating mode of an internal combustion engine in accordance with the preamble of claim 1.
The present disclosure describes systems and methods for recharging a storage catalyst of an internal combustion engine. A method, comprising: while operating an engine in a substoichiometric operating mode when the engine is under medium load and responsive to an LNT condition, assisting the engine with an electric machine connected to an engine crankshaft. The electric machine provides an auxiliary drive to assist the engine in maintaining the substantially steady state substoichiometric operating mode which may be used to reduce NOx or SOx build up in a storage catalytic convertor or to assay the condition of a storage catalytic convertor.
In the present disclosure, the exhaust gas is enriched with unburned hydrocarbons as a reducing agent by means of a substoichiometric operating mode (λ<1) of the internal combustion engine. However, further measures for enrichment may be provided.
After initiation of enrichment, the substoichiometric operating mode of the internal combustion engine, once established, with a substantially constant air ratio λ=constant is maintained by satisfying additional power demand by means of an electric machine, which can be connected to the drive train of the internal combustion engine. The electric machine may serve as selectable auxiliary drive when operating in the steady-state substoichiometric mode.
An internal combustion engine with the assistance of the electric machine may continue to operate in a steady-state mode. This ensures that the air ratio λ does not vary due to a change in operating parameters of the internal combustion engine. Transient operating conditions, under which the air mass and the fuel quantity have to be adapted to changed boundary or operating conditions, may be avoided.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. Further, the inventors herein have recognized the disadvantages noted herein, and do not admit them as known.